This invention relates to a method of providing temperature compensation for resistive bridge circuits, and to compensated bridge circuits when produced by the aforesaid method.
It is known to provide transducers which comprise four piezo-resistive strain gauges which are arranged as a Wheatstone bridge, the temperature co-efficients of resistance (TCR) of each of the piezo-resistive elements being substantially constant over the temperature range at which the transducer is required to operate. However, as a result of manufacturing tolerances the resistances of the four bridge elements are rarely equal at a zero value of the input quantity to which the transducer is intended to respond, the transducer output voltage at zero input being referred to as zero error. Additionally the TCRs of the bridge elements are rarely equal, so that the zero error also changes with temperature.
Additionally the transducer sensitivity, expressed as change in output voltage for a given change of input value and unit supply voltage across the bridge, also changes with temperature.
It has previously been proposed to reduce both the zero error, and the change in this error as a result of temperature change, by inserting two compensating resistances which are respectively in parallel and in series with selected piezo-resistive elements of the bridge circuit, the compensating resistances having very low temperature co-efficients of resistance. It has also been previously proposed to maintain the transducer sensitivity substantially constant by introducing resistor/thermistor networks into the bridge power supply and into each half of the bridge circuit on either side of the output connections. The components of the relatively complex networks required for the aforesaid sensitivity compensation interact with each other and with the zero-compensating resistances. The complexity of these known compensating arrangements makes it difficult or impossible to provide both zero-level compensation and sensitivity compensation such that the characteristics of the compensated transducer may accurately be predicted.